1. Field of the Invention
This invention relates to a portable infusion device which is carried externally or internally by the user and which constitutes a controlled release system capable of delivering a steady dose of drug or other pharmaceutical agent for a prolonged period.
2. Description of the Prior Art
Many kinds of parenteral drug therapy require continuous delivery in preference to single or multiple injections. Benefits that accrue from continuous therapy may include, for instance, reduction of toxic or other side effects associated with sharp pulses of drug, significant improvement in the effectiveness of the therapy, and increased comfort of the patient. The traditional manner of administering sustained parenteral treatments is an intravenous drip. While this may be perfectly acceptable in a hospital environment, it obviously imposes severe restrictions on the activity of the recipient. As a result, considerable research over the last few years has been devoted to the development of small portable infusion pumps. The simplest of these have clockwork or battery operated motors, which drive syringe or peristaltic pumps, and are strapped to the user with a harness. A typical example is described by A. J. Handley, in "Portable Heparin Injector", The Lancet, 2, p. 313 (1970). A more recent development in the art is the implantable miniature pump. Such pumps have been explored particularly for the treatment of diabetes by continuous insulin therapy, and in addition to peristaltic or syringe devices, others powered for instance by the elastic tension of an inflated rubber balloon or the vapor pressure of a liquified propellant have also been used. For a review of these pumps, see M. V. Sefton, "Implantable Pumps", Medical Applications of Controlled Release Vol 1, R. S. Langer and D. L. Wise (eds), CRC Press, Boca Raton FL (1984).
The disadvantages of these pumps are that they are quite complex and costly to manufacture, making them inappropriate for widespread application. Most of them are also designed to deliver relatively large quantities of fluid and do not dispense small volumes of the order of a few milliliters effectively.
At the other end of the scale a range of tiny osmotic pumps based on the principle of the Rose-Nelson pump described in detail below has been devised. These are activated by imbibition of water from the surrounding environment and are designed for implantation or ingestion into the body, such that the absorption of body fluids is the triggering mechanism. A typical example is disclosed by F. Theeuwes in U. S. Pat. No. 3,760,984 (September 1973). These pumps are available commercially from Alza Corporation of Palo Alto, Calif., under the name Alzet.RTM.. Theeuwes has also developed a further simplification of the Rose-Nelson concept which can be prepared by a simple tabletting machine. (U.S. Pat. No. 3,845,770 (November 1974)). These miniosmotic pumps and tablets are capable of delivering quantities of drug around a milliliter or so at dosage rates of the order of 1 .mu.l per hour. They are simple and cheap to produce, but are generally limited to internal use because of their need to reside in an aqueous environment.
The advantages of simplicity, reliability and small size offered by Rose-Nelson type devices have been employed in the art only to a limited extent in osmotic pumps intended for external use. U.S. Pat. No. 3,604,417 (September 1971) discloses a modification of the Rose-Nelson pump in which a movable piston replaces the elastic diaphragm separating the drug and salt chamber, and both the drug and salt are loaded into the pump as solutions. U.S. Pat. No. 4,474,048 discloses another modification employing an impermeable elastic wall, and a movable end wall which can be screwed in to deliver a pulse dose of the contained drug at any time during the operation of the pump. U.S. Pat. No. 4,474,575 is a variant of U.S. Pat. No. 4,474,048 where the flow rate of the dispensed agent can be varied by altering the area of the semipermeable membrane exposed to the water chamber.
U.S. Pat. No. 4,552,561 discloses a pump assembly for use with a small osmotic pump, which can be filled in advance of use with the active agent to be dispensed. The action of the pump is initiated by filling the lower chamber of the housing with a hydrogel. Once the pump is in action, an optional mechanism for delivering pulse doses can be employed.
All of the above mentioned patents disclose pumps which are self driven and begin to operate as soon as they are primed with the contents of the several chambers. However in many instances it would be desirable to load the pump and store it until needed. This would enable the pump to be dispensed by a pharmacist for example, instead of requiring installation in a hospital setting. Furthermore the ability to store the filled pump, distribute to potential users, and activate very rapidly on demand would extend the field of use considerably to include such important medical emergencies as attack by toxic agents in warfare, severe allergic reactions and so on.